core: Fixes NPE in ActivateVdsVDSCommand

Fixes NullPointerException caused by running
VdsManager.refreshCapabilities() with vds parameter set to null.

Change-Id: If5b27df8ff0f5e79fe90d1b19e604ab107741dc4
Bug-Url: https://bugzilla.redhat.com/991729
Signed-off-by: Martin Perina <mperina@redhat.com>